Multiple input multiple output (MIMO) systems are wireless communications systems that may transmit signals utilizing a plurality of transmitting antennas, and/or receive signals utilizing a plurality of receiving antennas. Communications between MIMO systems may be based on specifications from the Institute of Electrical and Electronics Engineers (IEEE). A MIMO system that receives a signal Y may compute a channel estimate matrix, H, based on the received signal. The signal may comprise information generated from a plurality of information sources. Each such information source may be referred to as a spatial stream. A transmitting MIMO system may utilize a plurality of transmitting antennas when transmitting a corresponding signal X. A receiving MIMO system may utilize a plurality of receiving antennas when receiving the signal Y. The channel estimate matrix for a downlink RF channel, Hdown, may describe a characteristic of the wireless transmission medium in the transmission path from a transmitter, to a receiver. The channel estimate for an uplink RF channel, Rup, may describe a characteristic of the wireless transmission medium in the transmission path from the receiver to the transmitter.
According to the principle of reciprocity, a characteristic of the wireless transmission medium in the transmission path from the transmitter to the receiver may be assumed to be identical to a corresponding characteristic of the wireless transmission medium in the transmission path from the receiver to the transmitter. However, the channel estimate matrix Hdown may not be equal to a corresponding channel estimate matrix for an uplink RF channel Rup. For example, a noise level, for example an ambient noise level, in the vicinity of the transmitter may differ from a noise level in the vicinity of the receiver. Similarly, an interference level, for example electro-magnetic interference due to other electro-magnetic devices, in the vicinity of the transmitter may differ from an interference level in the vicinity of the receiver. At a transmitter, or receiver, there may also be electrical cross-coupling, for example leakage currents, between circuitry associated with a receiving antenna, or a transmitting antenna, and circuitry associated with another receiving antenna, or another transmitting antenna.
The principle of reciprocity, wherein it may be assumed that Hup=Hdown, may also be based on the assumption that specific antennas at a transmitter or receiver are assigned for use as transmitting antennas, and/or assigned for use as receiving antennas. At the transmitter, a number of receiving antennas, NRX, utilized at the receiver may be assumed. At the receiver, a number of transmitting antennas, NTX, utilized at the transmitter may be assumed. If the assignments of at least a portion of the antennas at the transmitter are changed, the corresponding channel estimate matrix H′up may not be equal Hdown. Similarly, if the assignments of at least a portion of the antennas at the receiver are changed, the corresponding channel estimate matrix H′down may not be equal Rup. Consequently, after reassignment of antennas at the transmitter and/or receiver, the principle of reciprocity may not be utilized to characterize communications between the transmitter and the receiver when Hup does not equal H′down, when H′up does not equal Hdown, or when H′up does not equal H′down.
The principle of reciprocity may enable a receiving wireless local area network (WLAN) device A to receive a signal Y from a transmitting WLAN device B, and to estimate a channel estimate matrix Hdown for the transmission path from the transmitting WLAN device B to the receiving WLAN device A. Based on the channel estimate matrix Hdown, the WLAN device A may transmit a subsequent signal X, via an uplink RF channel, to the WLAN device B based on the assumption that the channel estimate matrix Hup for the transmission path from the transmitting WLAN device A to the receiving WLAN device B may be characterized by the relationship Hup=Hdown. When the WLAN devices A and B are MIMO systems, corresponding beamforming matrices may be configured and utilized for transmitting and/or receiving signals at each WLAN device.
Beamforming is a method for signal processing that may allow a transmitting MIMO system to combine a plurality of spatial streams in a transmitted signal X. Beamforming may comprise computing a matrix of beamforming coefficients. The beamforming coefficients may be utilized to compute a plurality of weighted sums representing a corresponding combination of signal strength levels from at least a portion of the plurality of spatial streams. Each weighted sum may be referred to as a radio frequency (RF) chain. A transmitting WLAN device may simultaneously transmit an RF chain from each of the plurality of transmitting antennas. The transmitted signal X may comprise the plurality of transmitted RF chains. Beamforming is also a method for signal processing that may allow a receiving MMO system to separate individual spatial streams in a received signal Y.
As a result of a failure of an assumed condition for the principle of reciprocity, a beamforming matrix at the transmitting WLAN device, and/or an equalization matrix at the receiving WLAN device, may be configured incorrectly. In a transmitted signal X, from the perspective of a signal associated with an ith spatial stream, a signal associated with a jth spatial stream may represent interference or noise. Incorrect configuration of one or more beamforming matrices may reduce the ability of the receiving WLAN device to cancel interference between an ith spatial stream and a jth spatial stream. Consequently, the received signal Y may be characterized by reduced signal to noise ratios (SNR). There may also be an elevated packet error rate (PER) when the receiving WLAN device decodes information contained in the received signal Y. This may, in turn, result in a reduced information transfer rate, as measured in bits/second, for communications between the transmitting WLAN device and the receiving WLAN device.
In some MIMO systems, a transmitting WLAN device may transmit a plurality of spatial streams based on channel state information at the transmitter (CSIT). The CSIT may be based on feedback information sent from the receiving WLAN device B to the transmitting WLAN device A. Based on the CSIT, the transmitting WLAN device A may compute estimated values for the channel estimate matrix Hdown.
Channel sounding is one method by which a transmitting WLAN device may receive CSIT from a receiving WLAN device. When performing a channel sounding procedure, the transmitting WLAN device may transmit one or more sounding frames to the receiving WLAN device. In some MIMO systems, the sounding frames may be transmitted without beamforming by a plurality of RF chains. In this respect, the matrix utilized for generating a plurality of RF chains from a plurality of spatial streams may comprise an identity matrix.
Upon receipt of a sounding frame, the receiving WLAN device may begin to compute channel state information (CSI). The CSI may be represented by the channel estimate matrix H. The CSI may be sent to the transmitting WLAN device as feedback information. The receiving WLAN device may not utilize beamforming when transmitting signals for sending the CSI to the transmitting WLAN device. The transmitting WLAN device may utilize the received feedback information to generate a beamforming matrix. The transmitting WLAN device may utilize the beamforming matrix to transmit one or more subsequent frames comprising data in a subsequent transmitted signal X. After receiving the subsequent frames, the receiving WLAN device may send an acknowledgement frame to the transmitting WLAN device. The receiving WLAN device may not utilize beamforming when transmitting signals for sending the acknowledgement frame to the transmitting WLAN device.
In some MIMO systems, the channel sounding procedure comprises time one or more time durations during which a plurality of RF chains may be transmitted without utilizing beamforming. During these periods, a transmitting WLAN device may not be able to transmit data from one or more spatial streams in a transmitted signal X, such that the receiving WLAN device would be able to generate estimates for each of the spatial streams in a received signal Y. In this regard, the amount of time required to perform the channel sounding procedure may result in a reduction in the information transfer rate between the transmitting WLAN device and the receiving WLAN device.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.